Table of content

NOAA GOES Satellite Mission GOES-8 and GOES-10 SEM - Space Environment Monitor A New GOES Platform How Satellites Are Named Data Cautions Magnetic field Magnetometer Data Quality Energetic particles Particle Data Quality GOES Energetic Particle Correction Algorithm The Upgraded Algorithm X-rays X-rays Data Quality Acronyms:

NOAA GOES Satellite Mission

GOES satellites provide the kind of continuous monitoring necessary for intensive data analysis. They circle the Earth in a geosynchronous orbit, which means they orbit the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to hover continuously over one position on the surface. The geosynchronous plane is about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view of the Earth. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop the GOES satellites are able to monitor storm development and track their movements.

GOES satellite imagery is also used to estimate rainfall during the thunderstorms and hurricanes for flash flood warnings, as well as estimates snowfall accumulations and overall extent of snow cover. Such data help meteorologists issue winter storm warnings and spring snow melt advisories. Satellite sensors also detect ice fields and map the movements of sea and lake ice.

GOES-8 and GOES-10

The United States normally operates two meteorological satellites in geostationary orbit over the equator. Each satellite views almost a third of the Earth's surface: one monitors North and South America and most of the Atlantic Ocean, the other North America and the Pacific Ocean basin. GOES-8 (or GOES-East) is positioned at 75 W longitude and the equator, while GOES-10 (or GOES-West) is positioned at 135 W longitude and the equator. The two operate together to produce a full-face picture of the Earth, day and night. Coverage extends approximately from 20 W longitude to 165 E longitude. This figure shows the coverage provided by each satellite.

GOES view of Earth

http://spidr.ngdc.noaa.gov/spidr/help/images/goesview.gif

The main mission is carried out by the primary instruments, the Imager ( http://140.90.207.25:8080/EBB/ml/gsensor.html ) and the Sounder ( http://140.90.207.25:8080/EBB/ml/gsensor1.html ). The imager is a multichannel instrument that senses radiant energy and reflected solar energy from the Earth's surface and atmosphere. The Sounder provides data to determine the vertical temperature and moisture profile of the atmosphere, surface and cloud top temperatures, and ozone distribution.

Other instruments on board the spacecraft are a Search and Rescue transponder, a data collection and relay system for ground-based data platforms, and a space environment monitor. The latter consists of a magnetometer, an X-ray sensor, a high energy proton and alpha detector, and an energetic particles sensor. All are used for monitoring the near-Earth space environment or solar "weather."

GOES-10 Characteristics
Main body:	2.0m (6.6 ft) by 2.1m (6.9 ft) by 2.3m (7.5 ft)
Solar array:	4.8m (15.8 ft) by 2.7m (8.9 feet)
Weight at liftoff:	2105 kg (4641 pounds)
Launch vehicle:	Atlas I
Launch date:	April 25, 1997 Cape Canaveral Air Station, FL
Orbital information:	Type: Geosynchronous Altitude: 35, 786 km (22, 236 statute miles) Period: 1,436 minutes Inclination: 0.41 degrees
Sensors:	Imager Sounder Space Environment Monitor (SEM) Data Collection System (DCS) Search and Rescue (SAR) Transponder

SEM - Space Environment Monitor

NOAA operates a series of meteorology observing satellites known as Geosynchronous Operational Environmental Satellites (GOES). Even though the weather pictures from GOES are seen nightly in our living rooms via the local weather broadcast, few people know that GOES also monitors space weather via its onboard Space Environment Monitor (SEM) system. The three main components of space weather monitored by GOES at 35,000 Km altitude are: X-rays, energetic particles, and magnetic field.

The GOES X-ray detector?s primary function is to provide a sensitive means of detecting the beginning of solar flares--explosive events on the Sun?s surface that are fueled by the intense magnetic fields that accompany sunspots. The larger solar flares can cause massive ejections of solar matter which reach all points in the solar system and are measured by GOES energetic particle sensors. Solar activity can also cause disturbances in the solar wind which can propagate to Earth and disturb our local magnetic field. The GOES on board magnetometer measures fluctuations near the boundary of that field which are used to correlate with the world-wide system of ground-based magnetometers.

http://spidr.ngdc.noaa.gov/spidr/help/images/corona.gif

Space Weather is dominated by the Sun which supplies "seasons" in the form of the solar cycles--cycles of solar storms that erupt with varying force and frequency over an 11-year period. Ever present "trade winds" come in the form of galactic cosmic rays, a wind of atomic nuclei that blows steadily from all points in the galaxy and moves with such velocity that the nuclei penetrate everything in their path and come to rest deep within the Earth?s crust. Earth?s atmosphere and magnetosphere interact with these greater forces in a complex manner that in some ways provides a measure of protection from them and in others carries their effects to all forms of space-borne and ground based technologies and biological systems.

NGDC archives most of the key parameters needed to study space weather and to increase our understanding of its physical dynamics. The GOES SEM archive is the corner stone of that understanding and is critical to the newly initiated National Space Weather Program --a interagency program to provide timely and reliable space environment observations and forecasts. Current Space Weather forecasts are inadequate because techniques are based on statistics rather than an understanding of how Space Weather really works. Meteorologists have demonstrated that significant improvements in forecasting come when timely and relevant measurements are used in numerical models based on sound physical principles. US business partners in the National Space Weather Program include operators and manufacturers of satellite systems, electrical power systems, navigation systems, communication systems and manned space flight systems.

The data are transmitted via direct telemetry to the Space Environment Center (SEC) in Boulder, Colorado where they are use in real-time alerts and space weather forecasts. At the end of each month these data are transferred to the Solar-Terrestrial Physics Division of the National Geophysical Data Center, an organization known internationally as World Data Center A for Solar-Terrestrial Physics.

A New GOES Platform

NOAA began making geosynchronous weather observations in July 1974 and the first GOES was launched for NOAA by NASA in 1975. This GOES was followed with another in 1977. The initial series of satellites maintained attitude control by spinning. With the advent of GOES-8, launched in 1995, the basic platform design was changed to one called "3-axis stabilized." This required changes in all of the GOES data collection systems. The data descriptions below mention details of the "spinning" SEM because it applies to data collected prior to GOES-8. Currently, the United States is operating GOES-8 and GOES-10, launched in 1997. GOES-9 (which malfunctioned in 1998) is being stored in orbit to replace either GOES-8 or GOES-10, should either fail.

• http://www.ssec.wisc.edu/data/index.html
• http://www.sec.noaa.gov/
• http://www.geo.nsf.gov/atm/nswp/nswp.htm

How Satellites Are Named

NOAA assigns a letter to the satellite before it is launched, and a number once it has achieved orbit. For example, GOES-H, once in orbit, was designated GOES-7, GOES-G, which was lost at launch, was never assigned a number.

For more detailed information about the GOES satellites, see the GOES I-M DataBook, Revision 1, published 4 January 1997 by Space Systems-Loral. Also the GOES Pamphlet published when GOES-8 was launched. The most recent pictures received from the directly from the NOAA GOES satellites can be found at the NOAA GOES Server.

• http://rsd.gsfc.nasa.gov/goes/text/goes.databook.html
• http://ns.noaa.gov/NESDIS/GOES_overview.html
• http://www.goes.noaa.gov/

Data Cautions

The database contains 5-minute and 1-minute averages of the X-rays, energetic particles, and magnetic field data collected by GOES-5, -6, -7, -8, -9 and -10 between January 1986 and September 1998.

The volume of these data makes it impossible to issue a guarantee as to the quality of each data point. A quality pass has been made though each file to identify values that make wild excursions from the norm, and instances of such have been looked at on a case by case basis and compared with concurrent data from other satellites. Data identified as bad have been replaced with the bad data flag. Users should be suspicious of ?spikes? in the data and attempt to correlate them with other sources before assuming that they represent the space environment.

The time of these observations has not been corrected for the down-link and preprocessing delays. The Space Environment Center estimates that delay to be 5-6 seconds.

Magnetic field

• Magnetic field vector (Hp, He, Hn)
• Hp Parallel to satellite spin axis,
• He Earthward,
• Hn Normal to Hp and He, points West for GOES 1-4, East for GOES 5+
• Ht Magnitude of total magnetic field vector

A twin-fluxgate spinning sensor allows Earth's magnetic field to be described by three mutually perpendicular components: Hp, He and Hn. Hp is parallel to the satellite spin axis, which is itself perpendicular to the satellite's orbital plane. He lies parallel to the satellite-Earth center line and points earthward. Hn is perpendicular to both Hp and He, and points westward for SMS-1, SMS-2, GOES-1, GOES-2, GOES-3, and GOES-4, and eastward for later spacecraft. He and Hn are deconvoluted from the transverse component Ht. Field strength changes as small as 0.2 nanoTesla can be measured.

The magnetometer samples the field every 0.75 seconds. Four of these values constitute a frame and are sent to the ground station together. For data from GOES-3 or earlier spacecraft, the high and low values in the frame were thrown out and the remaining value closest to the previous frame's value is recorded. For data from GOES-5 and later spacecraft the high and low values in the frame are thrown out and the average of the two remaining values is recorded. No record is kept of which of the four values are used in the archive.

Magnetometer Data Quality

The GOES-5 magnetometer HP component had an artificial offset from January 2, 1986 to March 13, 1986. The data are left as is. The GOES-6 magnetometer experienced irregularities in the magnetometer on September 9, 1991. The transverse component, which is deconvoluted into the HE and HN components (orthogonal to spin axis), began to yield bad values due most likely to an error in locating Earth?s limb. The problem persists to this time. Although the possibility exists that a proper deconvolution may be arrived at, the data for these values have been replaced with the bad data flag and will not be plotted.

In summary, the HE and HN components of the GOES-6 magnetometer have been filled with the bad data flag from September 9, 1991 onwards. The HP component is left intact. The GOES-7 magnetometer experienced instrument failure of its transverse component in May 1993. Only the HP component is available from May 1993 onwards. The HN and HE components are filled with the bad data flag. The absolute accuracy of HP (spin axis component) on all GOES can be uncertain because of difficulties in calibration.

Energetic particles

• Protons
• Alpha-particles
• Electron fluxes

Solid-state detectors with pulse-height discrimination measure proton, alpha-particle, and electron fluxes. The look direction of the EPS (Energetic Particle Sensor) is perpendicular to the GOES spin axis which is approximately aligned with Earth's rotation axis. Since the satellite spin period, 0.6 seconds, is much shorter than the accumulation times, the EPS provides a spin-averaged estimate of the local high-pitch-angle particle fluxes. The integral electron channel is given in units of count/cm2 sec sr while the other channels are given in count/cm2 sec sr MeV at the average energy.

Because GOES spacecraft travel in a geostationary orbit, the E1 and P1 channels are responding primarily to trapped outer-zone particles. The P2 channel may occasionally respond to trapped particles during magnetically disturbed conditions. The geomagnetic cutoff at geostationary orbit is typically of the order of a few MeV as indicated by the lack of trapped P2 response except as noted above. Therefore, the remaining proton and alpha particle channels measure fluxes originating outside the magnetosphere -- from the sun or the heart of the Galaxy.

Particle Data Quality

Users of GOES particle data should be aware that significant secondary responses may exist in the particle data, i.e. responses from other particles and energies and from directions outside the nominal detector entrance aperture. A description of the algorithm that partially corrects for these effects is described below.

The electron detector responds significantly to protons above 32 MeV; therefore, electron data are contaminated when a proton event is in progress. Beginning with GOES-8 the electron data have had a preliminary correction applied, however, even these data are not to be considered research quality at this time. The daily averages for electrons are filled with the bad data flag of -999 when more than three I4 (>30 MeV proton) values during that day exceed 10. If you are curious about when that happens, you may view the log . These changes were first posted online April 29, 1998.

The GOES-5 electron channel is noisy from 1986 onwards and readings are a possible factor of 2 high. One component of the GOES-6 particle detector system has had radiation damage since 1986 that reduced its counting efficiency progressively. At present the E1 and P4 channels derived from this component record at only a few percent of their proper rates. In 1991 the telescope component of the GOES-7 energetic particle detector system experienced episodes of malfunction (noise). The first period began at 0330 UT, October 18, 1991 and extended to November 5, 1991. The detector was commanded off for 12 hours. At turn-on the detector appeared to have recovered, but failed again on November 11, with a rerecovery on November 12 after a second turn-off of three hours. The detector has since operated normally. The noise periods may be identified by unusually high rates being shown by the P1 channel and the derived > 1 MeV integral channel. Currently, the GOES-7 Energetic Particle Sensor is left turned off for 4 hours after eclipse to minimize bad data.

GOES Energetic Particle Correction Algorithm

R. D. Zwickl NOAA Space Environment Center

In January 1990, an upgraded algorithm for calculating the energetic-particle differential and integral proton flux from measurements made by the energetic particle monitors onboard the GOES-6 and -7 satellites became operational in NOAA's Space Environment Center (SEC). The following is a brief description of the rationale for the new algorithm and its basic features.

Why Did We Need a New Algorithm?

The energetic particle monitors are simple solid-state sensors, designed to handle large count rates without overwhelming the electronics. Since their launch these instruments have met their design goals and have never saturated, even during the largest events. However, because they were required to measure high rates, the detectors were built with passive shielding (no anti- coincidence). This has allowed particles to pass through the shielding from any direction and be counted as though they had entered through the front collimator.

During solar energetic-particle events the low-energy passbands would detect particles at exactly the same time as the high-energy passbands did, even though it was impossible for the lower-energy particles to be present at such early times. During quiet times, cosmic rays and their secondary particles produce a very high background in the GOES sensors, in contrast to their effect on more advanced sensors that use active shielding (>100 times the "nominal" background).

The initial algorithm, used until January 1990, did not take either of those effects into account. (NGDC has since applied the correction algorithm to the earlier data from 1986 to 1990.)

The Upgraded Algorithm

The count rate as measured by any one of the seven energetic particle proton channels on GOES-6 or -7 (identical systems) can be given by

CMeas = CTrue + S + BG

where CMeas is the actual measured count rate, CTrue is the true count rate, S is the count rate generated by particles entering through secondary energy passbands (i.e., those particles not passing through the collimator), and BG is the background count rate (produced primarily by cosmic rays). Simply stated, the new algorithm solves for CTrue as follows:

CTrue = CMeas - S - BG

The first step in the algorithm is to determine the background count rate for each of the seven channels. Since the background varies with time, a filter technique is used to find a new minimum value within the previous 10 days or use the previous value. This background value is then subtracted from CMeas. It is then assumed that the energy spectrum of the energetic particles, from one energy channel to the next, can be represented by a simple power law in energy ( ), and that the secondary energy passbands that were determined during calibration are responsible for all of the secondary count rate. The resulting set of equations can then be solved, starting with the highest energy channel and working toward lower energies. All seven energy channels must contain data or no values are calculated.

Finally, each set of 5-minute-averaged values is calculated independently of every other set of values. This allows the corrected values to be calculated continuously in an operational environment.

X-rays

Two band X-Ray flux (XL, XS) XL 1 - 8 A X-rays XS 0.5 - 4 A X-rays

Ion chamber detectors provide whole-sun X-ray fluxes for the 0.5-to-4 and 1-to-8 A wavelength bands. These observations provide a sensitive means of detecting the start of solar flares. Two bands are measured to allow the hardness of the solar spectrum to be estimated.

X-ray photons pass through a collimator which defines the view aperture, followed by a thin metallic window which defines the low energy threshold, before entering the ion chamber. The XRS (X-RAY SENSOR) viewing direction is in the meridian of the spacecraft spin axis. Dynamic positioning of the XRS elevation provides for maintaining the sun in the swept field. The X-ray emission of the sun is determined once during each spin. The spin period is 0.6 seconds and the data for both bands are given in Watts/cm2 sec.

X-rays Data Quality

The X-rays sensors may experience significant bremsstrahlung contamination. This contamination is caused by energetic particles in the outer radiation belts and depends on satellite local time, time of year, and the local particle pitch- angle distribution. The X-ray sensors are also sensitive to background contamination due to energetic electrons that either deposit their energy directly in the telescope or strike the external structure and produce bremsstrahlung X-rays inside the ion chamber. Comparison of X-ray measurements from two concurrently operating GOES satellites reveals a systematic difference signal that shows both diurnal and seasonal variations. These variations are most noticeable when solar activity is low to moderate. Beginning with the GOES-8 detector the dynamic range of the instrument was shifted upwards to allow the highest flux events to be recorded. As a consequence of this, the lowest flux recordings are clipped.

Acronyms:

1. NOAA http://www.noaa.gov - National Oceanic and Atmospheric Administration
2. GOES http://www.sao.noaa.gov/goes/goestxt.html - Geosynchronous Operational Environmental Satellites
3. NGDC http://wwwngdc.noaa.gov - National Geophysical Data Center
4. SEC - Space Environment Center
5. SEM http://www2.ncdc.noaa.gov/docs/klm/html/c3/sec3-5.htm - Space Environment Monitor